I. Field of the Invention
The present invention is directed to a yoke apparatus for use with a rack-and-pinion steering system.
II. Description of the Prior Art
Automotive steering systems typically include a housing having a rack driven by a pinion gear. Rotation of a steering wheel turns the pinion gear. The pinion gear meshes with a plurality of teeth formed on the rack to drive the rack in one of two reciprocal directions. The rack in turn is connected to a pair of dirigible wheels. In addition, many automobile steering systems comprise a rotary control valve which is operable to supply pressurized fluid to move a double-acting hydraulic cylinder or actuator to assist translation of the rack.
In order to keep the teeth of the pinion gear and the rack in engagement, such steering systems employ a yoke apparatus. The yoke apparatus includes a bearing member (also referred to as a bearing disc) which is biased to force the rack towards the pinion gear. The bearing member has a pair of spaced apart bearing surfaces which slidingly contact the surface of the rack opposite the teeth of the rack. The bearing member is slidingly mounted in a housing bore which is formed in a nominally orthogonal manner with reference to the rack's intended position. This results in a nominal alignment of the bearing surfaces along an axis which extends coaxially with the axis of translation of the rack. A single biasing member, such as a coil spring or rubber O-ring, is mounted in the housing bore to force the yoke apparatus against the rack and bias the bearing surfaces in order to force the teeth of the rack against the teeth of the pinion gear. Thus, the yoke apparatus operates to nominally guide the rack along the axis of translation and hold the teeth of the rack and pinion in mesh during the application of torque to the pinion gear.
In practice, the use of an elastomeric or rubber O-ring as the single biasing member has been unacceptable. The O-ring suffers inelastic deformation ("creep") under pressure over a very short time, on the order of one or two years, and thus loses its ability to adequately bias the yoke against the rack. Moreover, with either a spring or an elastomeric O-ring as the single biasing member, it is not possible to maintain the axis of translation of the rack orthogonal to the axis of the housing bore. This is because of the tolerances involved in forming the housing bore, rack, and pinion gear. Accordingly, it has been found that the axis of translation of the rack may be angled with respect to the axis of the bearing surfaces of the bearing member, and may even undulate as a function of rotational motion of the pinion. When so misaligned, one end of each of the support surfaces engage the rack while opposite ends of the support surfaces are spaced away from the rack. As a further result, the bearing member itself may suffer angular misalignment within the bore and jam. In fact, such yoke assemblies may be said to be over constrained or to be of non-kinematic design.
For the above reasons, the rack is often held from smooth movement in one, or both, directions of travel. This is particularly so when the rack travels in a direction from the contacting ends towards the non-contacting ends of the support surface. The edges resist movement of the rack and the rack tends to hesitate and jerk in its movement. However, movement of the rack in an opposite direction tends to produce a smoother, less resistant movement. Frequently, the discontinuous or halting movement of the rack will be tactilly sensed by the driver.
The spring is located in an adjuster plug which is threadably inserted in the outer portion of the housing bore. During the assembly of the yoke apparatus, the adjuster plug is rotatably driven into contact with the bearing member with a nominal torque value of perhaps 50 in lbs. to provide a rotational position reference. Because of the above noted tolerances involved in forming the housing bore, rack and pinion gear, there results a soft contact between the adjuster plug and the bearing member, and thus an imprecise rotational position reference. For this reason, the adjuster plug must then be backed off by an angle in the order of 30.degree. in order to ensure interference free operation in the manner described above. This results in an indefinite stop position of the bearing member should a torque level be applied that is sufficient to overcome the spring bias.
In operation, rack-and-pinion assemblies are often subject to conditions known as "shock" and "rattle". Shock most often occurs when the dirigible wheels are subject to dissimilar impacts such as encountering a pothole or when crossing obliquely disposed railroad tracks or similar road surface discontinuities. One cause of shock is thought to be physical separation followed by abrupt contact between the bearing member and top surface of the adjuster plug. Rattle tends to be caused by lateral and rotational motions and resulting impacts within the housing by the yoke apparatus elements when the rack is subject to a succession of impacts when traversing an uneven surface such as a rough unpaved road or an open grassy field. In any case, shock and rattle are typically treated by tightening up various clearances or adding elastomeric guide elements, and as a last resort, by significantly increasing the biasing spring force. Unfortunately, increasing the biasing spring force (by selecting a spring with a high spring constant) undesirably increases the resistance of the rack to movement.